1. Field of the Invention
The present invention relates to a V-belt continuously variable transmission used in an automobile or the like.
2. Description of the Related Art
JP 3-181659A, published by the Japan Patent Office in 1991, discloses a V-belt continuously variable transmission (V-belt CVT). The V-belt CVT is provided with a primary pulley into which engine rotation is input, a secondary pulley that outputs rotation to drive wheels, and a V-belt that transmits the primary pulley rotation to the secondary pulley. A ratio of an input rotation speed to an output rotation speed (speed ratio) is regulated by adjusting a ratio of a contact radius between a pulley and a V-belt in the primary pulley to that in the secondary pulley (pulley ratio).
When maintaining the pulley ratio in JP3-181659A, an oil pressure (primary pressure) supplied to the primary pulley is controlled based on an input torque and the pulley ratio so as to be an oil pressure necessary for maintaining the pulley ratio. When changing the pulley ratio, a correction amount for the primary pressure necessary for achieving a target speed change speed is computed. The primary pressure is increased or decreased based on the correction amount, thereby achieving the target speed change speed.
However, the primary pressure is reduced during a downshift in JP3-181659A, and therefore there is a possibility that a torque capacity of the primary pulley in the V-belt is excessively reduced to cause the V-belt to slip.
An object of the present invention is therefore to provide a V-belt continuously variable transmission in which there is no belt slippage during a downshift.